Rising fuel prices and government mandates are driving light and heavy duty vehicle makers to use technologies that reduce both fuel consumption and emissions. It is estimated that only about 33% of the energy from fuel combustion in diesel engines is captured for vehicle operation, while only about 25% of the combustion energy in gasoline engines is used to power the drive train and accessories. In current engine designs, a large fraction of the combustion energy is lost as waste heat. One approach to fuel savings involves recycling waste engine heat, which can be converted into motive or electrical power within the motor vehicle.
One method of waste heat recovery is thermoelectric (TE) generation, whereby direct current (DC) electrical power can be derived from a TE generation element (e.g., an n-type semi-conductor plus a p-type semi-conductor) that is exposed to a thermal gradient. A series connection of TE generation elements forms a TE generation module. Several TE generation modules can be connected in a combination of series and parallel configurations to form a TE generator (TEG). An illustration of an exemplary electrical connection incorporating a TEG is shown in FIG. 1. As illustrated in FIG. 1, a plurality of TE generation modules 10 form a TEG 11, which is electrically connected, for example, to a vehicle's electrical bus 12 and energy storage system (e.g., battery) 13. The current flowing through the connection is depicted, for example, by an arrow and reference label I. Due to the possibility of fluctuations in the current and the voltage of the TEG 11, a DC/DC converter 14 can be used to maintain a line voltage within a range that is compatible with a vehicle's electrical system.
To properly operate, a TEG requires a heat source (i.e. a higher temperature) and a heat sink (i.e., a lower temperature). The temperature gradient created induces a flux of electrical carriers across the TE generation elements. For motor vehicles, the heat source is generally the heat available within the exhaust gas, and the heat sink is generally the coolant circulating within the radiator or an independent cooler system. TEGs have, therefore, been proposed at various locations in a vehicle's exhaust system. Accessible sites may include, for example, the exhaust tailpipe and, particularly for diesel engines, the exhaust gas recirculation (EGR) loop. TEG prototypes have been built, for example, with Bi/Pb-telluride and mounted on a vehicle's tailpipe. Such telluride-based modules have exhibited heat to electrical power conversion efficiencies up to about 10%. EGR loop TEGs are also under development, focusing mainly on skutterudite materials, which for this application may have efficiencies of about 3-10% dependent on the recycled fraction.
There are, however, various factors to consider when designing and implementing a TEG within an exhaust system. Such factors can include the available hot temperatures, the heat flow, the proximity of the heat source and sink to the TEG, the footprint of the TEG in view of the limited space available within an engine compartment or on the underside of a vehicle chassis, and the desire to minimize the mass added to the vehicle. A TEG added to the exhaust gas stream may further undesirably increase the pressure drop or back-pressure on the engine, thereby increasing fuel consumption. Consequently, various challenges may arise in using conventional TEGs in light of space requirements, and the resulting increase in mass and back-pressure.
It may therefore be desirable to integrate a TEG within existing exhaust gas after-treatment devices, such as, for example, catalytic substrates and/or particulate filters, to profit from high available temperatures (e.g., compared with tailpipe locations), high heat flux, reduce the number of components to be carried by the vehicle, and avoid additional back-pressure on the engine. Furthermore, after-treatment device operation windows are generally limited by the high temperatures (e.g., catalytic conversion and filter regeneration operation windows), which may lead to temperature gradients within the devices and thermo-mechanical durability-limiting associated stresses. Therefore, it may also be desirable to integrate a TEG within existing exhaust gas after-treatment devices to widen the operation windows of the after-treatment devices, while also maximizing waste heat recovery in the vehicle.